The present invention relates to a suspension bearing, and more particularly to a strut type suspension bearing.
A suspension bearing is interposed between an upper end portion of a strut shaft and a support portion on a vehicle body in a strut type suspension which constitutes one of suspension types for vehicles. In general, in an automotive suspension system, a steering operation requiring less steering effort is demanded when a vehicle is steered to turn (or when a steering wheel is turned), whereas when the vehicle is driven to run straight ahead (when the steering wheel rests at its neutral position), it is required that transverse oscillations of steering road wheels produced by irregularities in a road surface are not transmitted to the steering wheel to thereby prevent the deterioration of running stability.
A conventional strut type suspension bearing has a function to reduce rotational torque (rotational resistance) produced when a steering wheel is turned to steer the vehicle to turn through relative rotation of a pair of rings. However, even when the vehicle is driven to run straight ahead, the rotational resistance remains the same as that resulting when the vehicle is steered to turn, and therefore there is a tendency that the running stability is not maintained. Due to this, according to a conventional technique that has been accepted so far, a rotational torque varying construction (for example, a variable gear ratio mechanism which utilizes a planetary gear set and a motor) is incorporated in a steering mechanism so as to obtain a proper rotational resistance that should result when the vehicle is driven to run straight ahead. However, the construction is complex, causing a problem that the production costs are increased.